~/opencv_contrib/modules/xfeatures2d/src/boostdesc.cpp:673:20: fatal error: boostdesc_bgm.i: No such file or directory

Solution:
Check the /home/pi/Downloads/opencv-3.4.0/build/CMakeDownloadLog.txt log file, search for boostdesc_bgm.i keyword in the log file CMakeDownloadLog.txt, and find that the download of this file has failed.
A total of 11 files were found to be missing as follows:
boostdesc_bgm.i
boostdesc_bgm_bi.i
boostdesc_bgm_hd.i
boostdesc_lbgm.i
boostdesc_binboost_064.i
boostdesc_binboost_128.i
boostdesc_binboost_256.i
vgg_generated_120.i
vgg_generated_64.i
vgg_generated_80.i
vgg_generated_48.i

Download and upzip. put them in the path of /home/pi/Downloads/opencv_contrib3.4.0/modules/xfeatures2d/src/ is OK.
Then re-compile:

make clean
make